Riyadh, Saudi Arabia - Saudi Arabia, Qatar, and Oman led efforts to talk US President Donald Trump out of an attack on Iran, fearing "grave blowbacks in the region", a senior Saudi official told AFP on Thursday.

The Gulf trio "led a long, frantic, diplomatic last-minute effort to convince President Trump to give Iran a chance to show good intention," the official said on condition of anonymity, adding that dialogue was continuing.

Some personnel were moved out of a major US military base in Qatar on Wednesday. Staff at US missions in Saudi Arabia and Kuwait were warned to exercise caution as fears mounted of another major military attack, following last year's unprecedented bombing of Iranian nuclear sites.

Trump had repeatedly threatened to intervene in a nationwide protest movement that was met with a brutal crack down by Iran, which promised to retaliate with strikes on US military and shipping targets.

The Republican seemed to change course Wednesday, saying he had received assurances from "very important sources on the other side" that Tehran would not execute demonstrators, something later confirmed by Iran's foreign minister.

The Gulf efforts aimed to "avoid an uncontrollable situation in the region," the Saudi official said.

"We told Washington that an attack on Iran would open the way for a series of grave blowbacks in the region."

They added: "It was a sleepless night to defuse more bombs in the region... the communication is still underway to consolidate the gained trust and the current good spirit."

Another Gulf official said "the message conveyed to Iran has been that an attack on US facilities in the Gulf would have consequences on relations with countries in the region."